"at the top of my head"

I don't know how to use it.

I think you're thinking of "off the top of my head." It means "spontaneously" or "at this exact moment."

Example:

JOHN: "You have to go see that movie. The dialogue was really funny."
MARY: "What was so funny?...Tell me some of the lines.
JOHN: "I can't think of any off the top of my head, but I guarantee you it's good."

How about "nothig is off the top of my head"?

I agree entirely with mjd's posting.....it can't be improved on. So I don't think there is any more to be said. Except.....how did the phrase originate? Maybe it has an explanation like "raining cats and dogs" of which I do know the origin.